High pressure tubular reactor apparatus

ABSTRACT

In a high pressure tubular reactor containing a plurality of tubular sections interconnected in series by means of connection devices, which reactor contains one or more reaction zones, there is positioned within each reaction zone a single rupture disc device from about 24 to about 40 feet downstream from the reaction zone inlet.

BACKGROUND OF THE INVENTION

The polymerization of ethylene in the presence of oxygen and/or freeradical initiators in tubular reactors at high pressures andtemperatures is well known in the art. It is also well known to increasethe polymer productivity rate by introducing the initiator at more thanone point along the length of the reactor, thereby establishing as manyreaction zones within the tubular reactor as there are initiatorinjection points in the system.

Commercial size reactors are generally constructed of a plurality oftubular segments connected in series relation by blocks or otherconnection devices. The dimensions of the reactors include insidediameters broadly in the range of from about 0.5 to about 3 inchestypically between about 1 and about 1.5, and total lengths of from about800 to about 3000 feet or even longer. In order to confine the reactorsystem to an area of reasonable and practical dimensions, the tube isprovided with many bends, e.g. in the fashion shown in FIG. 1 of U.S.Pat. No. 4,008,049, hereby incorporated into this specification byreference.

The polymerization reaction is highly exothermic and causes a rapid risein the temperature along the length of a reaction zone until it reachesa peak when the initiator has been used up and polymerizationdiscontinues. Cooling in one form or the other is required to controlthe reaction temperatures within safe and desired limits and to reducethe temperature of the reaction mixture to a suitable initiationtemperature after which it is contacted with additional initiator in thesubsequent reaction zone. It is therefore the usual practice to employwater-cooled jacketed reactors and in addition to introduce relativelycool side streams of the ethylene feed along the length of the reactorin cooling zones located between the reaction zones. In addition to thebeneficial cooling effect achieved by the ethylene side streamintroductions, further yield advantages are obtained thereby.

There are pressure fluctuations occurring in these tubular reactorsemployed for the production of polyethylene resulting in temperaturechanges within the reactor. Some of these pressure changes areincidental to reactions taking place during polymerization, but otherpressure changes are purposefully employed to prevent accumulation ofpolymer on the interior walls of the reactor tube, these purposefulchanges being known as "bump cycles" and being effected by the operationof "let-down" valves at the exit end of the reactor. This bump cyclemay, for example, cause the reduction of pressure from 40,000 psi to35,000 psi, this being a drop of 5,000 psi which causes shiftings of thetemperature profiles throughout the reaction zones within the reactortube, e.g. as depicted in FIG. 3 of the aforementioned U.S. Pat. No.4,008,049.

Although in normal operations the reaction conditions in each reactionzone can be controlled rather precisely, a fortuitous upset in theinitiator/monomer ratio or failure of control instruments can cause thetemperature to rise within a reaction zone to levels where degradationof the product occurs and sometimes the degradation is so severe thatthe polyethylene product completely decomposes into carbon and hydrogen.The decomposition causes a dangerous and rapid increase in pressure tolevels where the reactor tube might burst. It was generally believedthat polyethylene decomposition occurred in the form of an explosion ofconsiderable force involving pressure increases of up to about 500,000psi per second. In order to minimize fire and explosion hazards toprevent serious damage to the equipment in case of a decomposition, asthe pressure front proceeds within the reactor tube it has been theusual practice to install a multitude of rupture discs at regularintervals along the total length of the reactor tube. It was consideredessential that the rupture discs were located immediately before thebends of the reactor tube to allow the pressure front to proceed in astraight line and be released through the rupturing disc without achange in direction. For this purpose the rupture discs were installedwithin connection blocks of a modified Y shape in a positionperpendicular to the flow direction. Such blocks are shown in FIG. 1 ofthe aforementioned U.S. Pat. No. 4,008,049, e.g. by numerals 40 and 50,and the rupture discs were installed within the unconnected horizontalextension or arm of the Y block.

Although the aforementioned rupture discs function well in case ofdecomposition, the experience has been that in many instances a discwould rupture without any apparent reason for the failure, i.e. therewere no indications of reactor condition upsets or degradation of thepolymer product at the time of the rupture. In view of the considerablecapital cost for each rupture disc installation and associatedequipment, such as stacks and the production losses occurring during"down times" of a reactor it is therefore desirable to minimize both thenumber of rupture disc installations as well as the number of failuresof the discs for causes other than polymer decomposition.

It is therefore an object of the present invention to provide a tubularhigh pressure polymerization reactor having a minimum of rupture discswithout sacrificing safety. Another object of the invention is toprovide a tubular high pressure reactor having rupture discs positionedin specific locations to minimize or obviate failures due to causesother than decomposition of polymer.

A further object of the invention is to provide a novel rupturedisc-connection block assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of a connection block adapted for a rupturedisc installation.

FIG. 2 is a sectional view of the connection block taken on line A--A.

FIG. 3 is a sectional view of the connection block taken on line B--B,also showing a complete rupture disc assembly.

FIG. 4 diagramatically illustrates a portion of a polyethylene reactorincluding the improvements of this invention.

THE INVENTION

In accordance with the present invention there is provided a highpressure tubular polymerization reactor having at least two reactionzones, comprising a plurality of tubular sections, connection devicesbetween adjacent tubular sections, means for introducing monomer feed tothe inlet of each of said reaction zones; separate means for introducingpolymerization initiator to the inlet of each of said reaction zones,and associated with each reaction zone a singular rupture disc disposedwithin a connection device positioned downstream of said inlet at amaximum distance of about 40 feet therefrom.

Contrary to the previous theory discussed before, it has now been foundthat when decomposition of polyethylene occurs in a reaction zone, it isin the form of deflagration which appears to be focused around the pointof initiator introduction or in the vicinity thereof. A flame fronttravels from this point in both directions at a relatively slow speed,i.e. about the speed of sound. It has also been found that failures ofthe rupture discs for causes other than decomposition are to a greatextent caused by excessive cyclic temperature and pressure stressesimposed on said rupture discs, i.e. the disc failures are due to fatiguerather than over pressure. It is therefore important that the rupturediscs be located where the combination of cyclic pressure andtemperature stresses are minimized. These cyclic pressure andtemperature stresses mainly include those caused by the bump cyclesreferred to hereinabove, i.e. the deliberate periodic reduction ofreactor pressure which causes a shift in the temperature profile asdepicted in FIG. 3 of U.S. Pat. No. 4,008,049 (from profile 110 toprofile 112 in zone number 1). Minimum cyclical stresses occur in thereaction zone where the fluctuation in temperature is at a minimum.

Since the pressure fluctuations are deliberate, it follows that theminimum cyclical stress conditions are prevailing at a location wherethe temperature fluctuations are at a minimum. The rupture disc deviceshould therefore be positioned as close to the initiator introductionpoint as possible.

However, these high pressure reactors are manufactured frominterconnecting thick-walled, flanged tubular segments, which are boltedtogether by means of massive connection blocks or other fatigueresistant fittings. Each of these blocks or fittings usually also servesas an access port to the reactor, e.g. as instrumentation ports, orintroduction points for initiator streams, monomer side streams,modifiers, etc. Each segment of the reactor tube is surrounded by aseparate cooling jacket provided with jacket water bypass conduitsaround the connection devices to provide flow of coolant seriallythrough adjacent jackets. Although it would be desirable to position therupture disc in the same connection device used for initiatorintroduction or in an adjacent connection device, it is not a feasiblesolution considering the additional stress concentration effects imposedby such arrangements.

In view of the above-mentioned considerations which must be given to thedesign of a high pressure tubular reactor for the polymerization ofethylene, the rupture disc is suitably positioned a short distancedownstream from the initiator inlet, i.e. in the subsequent connectiondevice used in joining the first section of the reaction zone with thenext section. This distance should not exceed about 40 feet andpreferably is from about 24 to about 40 feet, most preferably from about28 to about 33 feet.

The disc or diaphragm is positioned within said connection device in alateral bore which is perpendicular to the longitudinal passage or boreproviding flow of reactants between two adjacent tubular reactorsegments. For the purpose of this application, the usual definition ofreaction zone is intended, i.e. a reaction zone begins at the pointwhere contact of monomer and fresh initiator occurs and ends where themaximum temperature peak is measured during a bump cycle.

In accordance with this invention the number of rupture discs installedthroughout the total length of the reactor tube is exactly the same asthe number of reaction zones established within the tubes by theseparate introduction of initiator.

Rupture discs are commercially available for any desired service andtherefore the choice of material of construction, signing, etc. need notbe discussed for the understanding of the invention.

The connection device should be fabricated from high strength metal suchas high grade alloy steels and the like.

The rupture disc can be positioned and installed in the block in anyknown fashion, however, a particularly advantageous rupturedisc-connection block assembly is shown in FIGS. 1-3. In such an asemblythere is a minimum of volume in which during down-times and purging ofthe reactor some impurities such as small traces of air, could beentrapped, which impurities could affect the product quality or evenpromote complete decomposition of the reaction mixture during subsequentoperations.

Referring to the drawings in FIG. 1, 2 and 3 there is shown arectangular block 1 provided with longitudinal bore 2, recesses 3adapted to receive a sealing cone ring at each end (not shown) andrecesses 4 adapted to receive threaded stud bolts (not shown). On eachside and aligned with the longitudinal bore there is a flanged sectionof a tubular reactor attached in sealing contact by means of the studbolts, sealing ring and nuts (not shown). Perpendicular to thelongitudinal bore there is a lateral bore 5, which at its lower portion6 has a diameter substantially equal to that of the longitudinal bore,and at its upper portion 7 has a diameter substantially larger than thelongitudinal bore. A ring shaped seat 8 is thereby provided, which, ifdesired, is beveled at its inner edge 9 as shown in FIG. 2 for bettersealing contact with the inserted rupture disc holder. Recesses 11located in the upper portion of the block are adapted to receivethreaded stud bolts 12 shown in FIG. 3.

Supported by seat 8 and in sealing contact at least at the inner edge ofthe seat there is a close fitting rupture disc holder 15, which isremovably positioned within the lateral bore. The upper portion of theholder extends upwards from the seat to near the upper surface of theblock thereby forming a recess 13 adapted to receive sealing cone ring14, while the lower portion of the holder extends downwardly into thelower portion 6 of the lateral bore. To achieve better sealing contactbetween the holder and the seat 8 the necked-in area 16 of the holder isslightly angled. Throughout the holder there is located a centrallateral bore 17, the diameter of which is gradually widened into aninverted frustoconical space 18 and preferably widened at the uppermostportion 19 to provide improved sealing contact with seal ring 14 whenfully assembled. A support ring 21 is affixed, such as by welding, tothe bottom portion of the holder and finally a rupture disc (diaphragm)22 which is fixedly attached to the support ring, is positioned at thejunction of the lateral bore and the uppermost portion of thelongitudinal bore.

Sealing contact is achieved by means of flange 23, sealing ring 14, studbolts 12 and nuts 24. Within the flange 23 there is a threaded adapter26, which serves to conduct escaping material from the reactor in caseof failure through subsequent piping (not shown) to a stack for ultimaterelease to the atmosphere.

In case of failure causing the diaphragm to rupture, the flange and sealring are removed, a removal tool is screwed into threaded holes 27, theholder is removed from the block, the support ring-rupture disc assemblyis cut off and a new such assembly welded on to the bottom of the holderbefore reassembly of the components.

Referring now to FIG. 4, which is related to FIG. 1 of U.S. Pat. No.4,008,049, there is shown a portion of an ethylene polymerizationreactor, specifically a first reaction zone and part of the reactionzone adjacent to the first one. The first and second reaction zones aremade up from tubular sections 30a and 30b respectively and joined bymeans of connection devices 31, 1 and 33. Connection devices 33 (2shown), through which polymerization initiator is introduced into theinlet of each reaction zone, join adjacent reaction zones to each other.Rupture discs are disposed within the connection devices 1, which areimmediately subsequent to connection devices 33.

The present invention is not limited to any particular tubular design,operating conditins, reactants or initiators. Generally, however, theinner diameters of the tubular segments in the reactor range betweenabout 0.5 to about 3 inches and the reactor length between about 800 andabout 3,000 feet or more. The reactor can be operated at pressures fromabout 15,000 to about 100,000 psi, preferably between about 30,000 andabout 50,000 psi. The reaction temperatures generally range from about250° F. to about 650° F. or higher.

The initiator for the polymerization reaction includes oxygen and theperoxides such as hydrogen peroxide, 2,4-dichlorobenzoyl peroxide,caproyl peroxide, lauroyl peroxide, t-butyl peroxyisobutyrate, benzoylperoxide, p-chlorobenzoyl peroxide, diisopropyl peroxydicarbonate,acetyl peroxide, decanoyl peroxide, t-butyl peroxypivalate, t-butylperoxyacetate, t-butyl peroxybenzoate, cumyl peroxide, diethyl dioxide,t-butyl hydroperoxide, methyl ethyl ketone peroxide, di-t-butyldiperoxyphthalate, hydroxyheptyl peroxide, cyclohexanone peroxide,p-menthane hydroperoxide, pinane hydroperoxide, cumene hydroperoxide,t-butyl peroxide, 2,5-dimethyl hexane-2,5 dihydroperoxide, t-butylperoctoate, t-butyl peracetate, 1,1,3,3-tetramethyl butyl hydroperoxide,or mixtures thereof.

In addition to the ethylene feed, a comonomer in amounts ranging from0.1 to 20 mole percent of the ethylene feed may be employed.Illustrative examples of such comonomers include alpha-olefins such aspropylene, butenes and pentenes, and other comonomers such as vinylacetate and the like. A chain transfer agent can also be introduced intothe polymerization system with the feed in amounts ranging from 0.01 to5 mole percent of the ethylene feed. The chain transfer agent includes,for example, hexane or butane or a comonomer such as propylene whichalso functions as a chain transfer agent. The addition of a comonomerand/or a chain transfer agent permits one to vary the physicalproperties of the polyethylene products as is well-known in the art.

What is claimed is:
 1. A high pressure tubular polymerization reactorcontaining a plurality of bends and having a least two reaction zones,comprising:a plurality of tubular sections each having an insidediameter in the range of from about 0.5 to about 3 inches; connectionblock devices between adjacent tubular sections; means for introducingmonomer feed to the inlet of each of said reaction zones; separate meansfor introducing polymerization initiator to the inlet of each of saidreaction zones, and associated with each reaction zone a singularrupture disc disposed within a connection block device positioneddownstream of said inlet at a distance of from about 24 to about 40 feettherefrom, wherein the connection device containing the rupture disccomprises a rectangular block having an upper surface, a bottom surfaceand two pairs of opposing side surfaces, said block being provided witha longitudinal bore extending from one side surface to its opposingsurface, and a centrally positioned lateral bore in relation to andextending from said longitudinal bore upwards through the upper surfaceof the block, the diameter of the lower portion of the lateral borebeing about equal to that of the longitudinal bore and the diameter ofthe upper portion of the lateral bore being substantially larger thanthat of the longitudinal bore, thereby providing a seat having a ringshape cross sectional area within the block; supported by said seat andclosely fitting within said lateral bore a removable necked rupture discholder having a larger diameter upper portion and a smaller diameterlower portion, said upper portion of said holder extending upwards fromsaid seat to near the upper surface of the block, thereby providing arecess in the upper surface of the block, said lower portion of theholder extending downwardly from said seat into the lower portion of thelateral bore of the block, said holder being provided with a lateralbore having a relatively small diameter in the lower portion of theholder and a relatively large diameter in the topmost portion of theholder; a support ring affixed to the lower portion of the holder andhaving about the same cross sectional dimensions as those of said lowerportion of the holder; a rutpure disc affixed at its periphery to thesupport ring and positioned at the junction of the lateral bore of theblock and the uppermost portion of the longitudinal bore; means forproviding sealing contact between the rupture disc holder and the seatwithin the block.
 2. The reactor of claim 1 wherein said distance isfrom about 28 to about 33 feet.
 3. The reactor of claim 1, wherein theseat within the block is beveled at its inner edge.
 4. The reactor ofclaim 1, wherein the neck of said holder is slightly angled.
 5. Thereactor of claim 1, wherein the upper portion of the lateral bore of theholder has an inverted frustoconical shape.
 6. The reactor of claim 1,wherein the inside edge of the holder at its uppermost portion isbeveled.
 7. The reactor of claim 1, wherein the means for providingsealing contact between the rupture disc holder and the seat within theblock comprises a flange having a recess within its lower centralportion, a seal ring seated in the recesses of the flange and the uppersurface of the block, said flange being removable attached to saidblock.
 8. The reactor of claim 7, wherein hollow adapter means isprovided within the flange for conducting material escaping from thereactor through a ruptured disc to a stack.